Actually, researchers have said that with a scaled up version of the station we would be able to send a full-sized, 100 kg spacecraft to Mars from the surface of the Earth in a matter of days. So it's not entirely impossible. https://mic.com/articles/135954/nasa-laser-propulsion-system-could-send-ships-to-mars-in-just-days#.HzJEd7Glm
My initial idea was to connect systems of planets with trade and transportation. Thus creating aspects within the universe such as sectors or "countries" per se. I do acknowledge your fear, but I believe that within this system there will be still people who go out to explore and create their own systems throughout the cosmos.
As for your idea on the battle aspect, I think this would be really awesome and great mechanic.